Lasers work on the principle of stimulated emission. This requires light of the correct frequency to impinge on excited atoms in the lasing region. In order to get an efficient laser the light must stimulate emission from as many of the excited atoms as possible. Conventionally, this is achieved by using a resonant cavity with mirrored ends so that the stimulating light undergoes multiple reflections and makes multiple passes of the lasing region. This arrangement results in a typical efficiency of only a few percent as much of the energy is lost out of the sides of the laser cavity and the mode in the cavity is unstable. In order to obtain even this low level of efficiency, the resonant cavity must be accurately produced in order to ensure that a standing wave is set up.
Some potential laser materials are not able to lase using a conventional apparatus because the lifetime of the excited states are too short to allow a population inversion to be sustained. It would be desirable to find an alternative arrangement that allowed these materials to lase.
Recently, work has been carried out investigating the lasing properties of random media, such as a powdered lasing glass. Random media of this sort give rise to strong scattering and interference which can act to trap light or at least strongly localise it. The multiple scattering events can be used to stimulate many atoms in a single pass of the material. It is therefore possible to use such random media in lasers without the need for resonant cavities.
However, there are problems associated with such random media. They are difficult to define and replicate and can give rise to anisotropic behaviour. Furthermore, it is difficult to predict the localising wavelength in a random structure. It is often specific to a particular wavelength and a particular direction of propagation.